Magnetic fluid display device

ABSTRACT

A fluid display device includes a housing comprising a pair of blocks joined at first surfaces. One of the blocks is transparent and the other is opaque. A plurality of display elements is provided in the device, each such display element including a plurality of cavities formed in the first surface of the transparent block. An opaque magnetic liquid having a predetermined surface tension characteristic and a clear, immiscible liquid fill the cavities, respectively. A light reflecting layer on the first surface of the opaque block is aligned with one of the cavities in the transparent block for viewing through the last-mentioned block from the second surface thereof. Channels are provided between the cavities to permit the flow of the magnetic fluid therebetween and separate passages are provided which extend between the non-adjacent ends of the cavities for communication therebetween, through which the clear fluid is transferred between the cavities. An electromagnet assembly is mounted in the second surface of the opaque block and includes a plurality of coils wound about a core. Core legs extend toward the cavities, predetermined ones thereof being adjacent predetermined ones of the cavities. Energization of selected coils moves the magnetic fluid between the cavities to block or reveal the light reflecting layer, respectively, thereby to change the state of the display element of the device. The force of the moving magnetic fluid forces the clear liquid through the passages for transfer to the other cavities. Each element has two conditions and remains in each of these conditions without provision of power for maintenance. The surface tension of the magnetic fluid and the size of the channels and passages, chosen accordingly, ensures the bistability of the elements. The device is position insensitive as well. The state of the elements of the device may be determined also without viewing, since the permeability of the magnetic core portion adjacent the cavities whereat the magnetic fluid is located, is different than that of the core portions adjacent those cavities having no magnetic fluid located therein. This changes the inductance in an associated coil. The sensing of the impedance of the coil thus provides a determination of the state of the element.

a t; I lilmte Stats Patent 1 m1 3,863,249 Matt Jan. 28, 1975 MAGNETHC FLUTE) DISPLAY DEVICE from the second surface thereof. Channels are pro- 1 Inventor Emer E Olah Schuumburg m vided between the cavities to permit the flow of the y magnetic fluid therebetween and separate passages are [73] Assignee: Motorola. nc-l g provided which extend between the non-adjacent ends z 197 of the cavities for communication therebetween, [22] Filed July 30 3 through which the clear fluid |s transferred between [2]] Appl. No.: 383,938 the cavities. An electromagnet assembly is mounted in the second surface of the opaque block and includes a [52] U S C] 340/373 140/336 plurality of coils wound about a core. Core legs extend [51] H dd 6 toward the cavities, predetermined ones thereof being [58] Field 336 373 R 324 adjacent predetermined ones of the cavities. Energiza- [56] References Cited UNITED STATES PATENTS 3,648.26) 3/1972 Rosenweig 340/373 R Primary Examiner-Harold l. Pitts Attorney, Agent, or Firm-Donald J. Lisa; Vincent J. Rauner [57] ABSTRACT A fluid display device includes a housing comprising a pair of blocks joined at first surfaces. One of the blocks is transparent and the other is'opaque. A plurality of display elements is provided in the device, each such display element including a plurality of cavities formed in the first surface of the transparent block. An opaque magnetic liquid having a predetermined surface tension characteristic and a clear, immiscible liquid flll the cavities. respectively. A light reflecting layer on the first surface of the opaque block is aligned with one of the cavities in the transparent block for viewing through the last-mentioned block ia dlljli tion of selected coils moves the magnetic fluid between the cavities to block or reveal the light reflecting layer, respectively, thereby to change the state of the display element of the device. The force of the moving magnetic fluid forces the clear liquid through the passages for transfer to the other cavities. Each element has two conditions and remains in each of these conditions without provision of power for maintenance. The surface tension of the magnetic fluid and the size of the channels and passages, chosen accordingly, ensures the bistability of the elements. The device is position insensitive as well. The state of the elements of the device may be determined also without viewing, since the permeability of the magnetic core portion adjacent the cavities whereat the magnetic fluid is located, is different than that of the core portions adjacent those cavities having no magnetic fluid located therein. This changes the inductance in an associated coil. The sensing of the impedance of the coil thus provides a determination of the state of the element.

13 Claims, 15 Drawing Figures PATENIED JAN28I97S SHEH 3 BF 5 MAGNETIC FLUID DISPLAY DEVICE BACKGROUND This invention relates generally to display devices and more particularly to fluid display devices.

Many different types of fluid display devices are available for use today. Some of these devices use colored liquid which is pumped selectively into predetermined chambers or cavities arranged in a particular pattern to produce various effects. An example of such a pattern is one in which seven compartments are arranged in the form ofa numeral 8. By selectively filling the compartments with the colored liquid, all of the numerals from to 9 can be formed.

In presently available liquid display devices, the liquid is either pumped into and out of the compartments or in some instances the liquid includes magnetic particles which are suspended therein, and as such, can be moved by magnetic attraction or repulsion. Such a magnetic fluid display device is described in U.S. Pat.

No. 3,648,269. This patent illustrates a display device in which magnetic liquid is used in conjunction with an immiscible transparent fluid, the latter of which wets the walls of the display container to enhance the movement of the magnetic fluid between cavities in the container.

For the most part, in these last-mentioned devices, when it is desired to maintain the liquid in a compartment for any period of time, that which provides the force to move the liquid; i.e. the pumps or electro or permanent magnets, must be maintained in operative relation with respect to the fluid. This often requires considerable energy. Furthermore, if for some reason the source of energy should be removed, the displayed pattern would be lost.

Furthermore, many of these devices are position sensitive; i.e. they must be oriented in a given manner otherwise they'will not function properly, and the speed of operation thereof is generally relatively slow.

In addition to the above, it would be desirable to be able to employ liquid display devices in electronic calculator and the like machines. However, in such an environment, the state of the liquid display device must not only be obtainable by observation, but also by sensing circuitry, etc., within the calculator machine itself.

SUMMARY Accordingly, it is a primary object of the present invention to provide a new and improved liquid display device which requires no external force or energy to maintain the device in a preset condition.

It is another object of the present invention to provide a liquid display device of the above described type which operates efficiently regardless of the position or orientation thereof.

It is yet another object of the present invention to provide a new and improved liquid display device which uses a two component fluid, one being an opaque fluid containing suspended magnetic particles and the other being an immiscible transparent fluid.

It is still another object of the present invention to provide a new and improved liquid display device of the above described type which is efficient in operation, relatively simple in construction and which operates at a greater speed than presently available fluid display devices.

It is yet another object of the present invention to provide a new and improved liquid display device which lends itself for use in electronic calculator machines wherein the state or condition of the device may be obtained by sensing circuitry provided therein.

Briefly, a preferred embodiment of the fluid display device according to the invention includes a housing comprised of a pair of joined blocks, preferably of a plastic material. One of the blocks is transparent and includes a plurality of fluid display elements. Each such element includes, formed in a first surface thereof, a plurality of cavities, preferably three in number, arranged side by side. The outer two cavities are joined to the central cavity along adjacent edges thereof by channels provided therebetween and separately by passages extending between non-adjacent edges thereof, respectively, to provide a circuitous path therebetween. An opaque liquid having magnetic particles suspended therein and a transparent liquid having no magnetic particles, fill the cavities. The transparent fluid is immiscible and wets the surface of the cavities, leaving a film or residue thereon. The latter makes it possible to move the fluids between cavities or compartments with complete displacement of the opaque liquid. For a better explanation of the type of liquids described herein, the readers attention is directed to US. Pat. No. 3,648,269.

One of the surfaces of the opaque block is sandwiched against the first surface of the transparent block. The opaque block includes a light reflecting portion which is aligned with one of the cavities in the transparent block, preferably the center one, so that this portion can be viewed through the transparent block from the opposite surface thereof.

An electromagnet assembly is provided adjacent the liquid filled cavities. The assembly includes a magnetic core having a main body portion with a plurality of legs extending therefrom spaced therealong. Windings are wound on the core between the core legs. Selective energization of the windings produces predeterminedly defined magnetic fields which move the magnetic and transparent liquids through the channels and passages between cavities. When the magnetic liquid is moved, the transparent liquid is forced from the cavity in which it was located to permit the magnetic liquid to move thereinto. The transparent liquid thus must move into the cavities originally occupied by the magnetic liquid. ln the case wherein the magnetic liquid is moved into the viewable cavity, the light reflecting portion is blocked and the cavity appears opaque. The other cavities also appear opaque at this time because of the opaque block which is viewed through the transparent block.

Once oriented, the winding(s) may be deenergized and the liquids will remain in their new locations, regardless of the movement or orientation of the display device. The surface tension of the magnetic liquid which in a preferred embodiment is approximately that of water, maintains the liquids in respective cavities after removal of the magnetic field. The magnetic liquid tends to orient itself in the shape of a sphere and because the passages joining the cavities are sufficiently small, they permit the flow of the liquids therethrough only under pressure caused by the energization of the electromagnets. The surface tension of the transparent liquid is less than that of the magnetic liquid.

If used in an electronic calculator machine, the state or condition of each element of the display device may be sensed easily by suitable circuitry. This can be accomplished because of the change in permeability of the magnetic core section adjacent the cavity filled with the'magnetic fluid. The inductance of the coil wound about this section becomes greater than the inductance of the other coils of the electromagnet assembly associated with the element and alters the impedance of this coil. To sense the impedance, an alternating current is passed through the coil. This information can be used in an electronic calculator machine to provide an indication of the condition of each of the elements. The bistability of the elements along with this sensing technique provides a memory for the calculator machine.

DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a perspective view ofa liquid display assembly comprising a plurality of liquid display devices according to the invention;

FIG. 2 is an enlarged, sectional perspective view of one of the elements of one of the liquid display devices of FIG. 1',

FIG. 3 is a plan view ofthe transparent block portion of a liquid display device according to the invention, illustrating the cavities formed therein for holding the liquids used in the device;

FIG. 4a is a plan view of the transparent block portion of FIG. 3 illustrating in detail one of the display elements of the device and the decimal point element thereof both of which include magnetic and transparent liquids filling the cavities comprising the elements;

FIG. 4b is a sectional view of the display element described in FIG. 4a taken along the line 4b4b and including additionally the opaque block portion and electromagnet assembly for changing the state of the display element, according to the invention;

FIG. 5a is a plan view of the block portion of FIG. 4a illustrating the transfer of the magnetic and transparent liquids between cavities of the one element upon operation of a predetermined coil(s) of the electromagnet assembly thereof;

FIG. 5b is a sectional view of the element of FIG. 4b, illustrating the transfer of liquids between cavities as shown in FIG. 5a;

FIG. 56 is a plan view of the core pieces of the electromagnet assemblies of the liquid display device of FIG. 4a illustrating the polarities of the core pieces of the one element when transferring the liquid as illustrated in FIGS. 50 and 5b;

FIG. 6a is a plan view of the liquid display device of FIG. 4a subsequent to the transfer of the liquids between cavities;

FIG. 6b is a sectional view of the element of FIG. 4b subsequent to the transfer of the liquids between cavities;

FIG. 7a is a plan view of the device of FIG. 4a illustrating the transfer of the liquids in the one element back to the original cavity locations;

FIG. 7b is a sectional view of the element of FIG. 4b illustrating the transfer of the liquids back to their original cavities;

FIG. 7c is a plan view of the core pieces of the electromagnet assemblies of the liquid display device of FIG. 4a illustrating the polarities of the core pieces of the one element when transferring the liquid as shown in FIGS. 7a, 7b,-

FIG. 8 is a sectional view of the decimal point element of the display device of FIG. 5a taken along the line 88, with the addition of the opaque block and electromagnet assembly provided for the decimal element, illustrating the transfer of the liquids between cavities therein; and

FIG. 9 is a sectional view of the decimal point element of the liquid display device of FIG. 7a taken along the line 9-9, illustrating the transfer of liquids back to their original cavities.

DETAILED DESCRIPTION Referring now to the drawings in greater detail wherein like numerals have been used throughout the various views to illustrate similar components, in FIG. 1 there is shown a display arrangement 10 comprising a plurality of individual fluid display devices 12 according to the invention placed in a side-by-side configuration. Each of the particular fluid display devices as shown, takes the form of a numerical display including seven elements, such as 14, which can be selectively activated to produce any number from 0 to 9. It will be apparent, however, that other display symbols, such as, for example, an alphabetic display, etc., can be provided if desired and still fall within the scope of the present invention.

The preferred embodiment of the fluid display device I2 according to the invention includes a housing 15 formed ofa pair of blocks 16, 18 joined at first surfaces thereof, both of which are fabricated from plastic or other suitable material. The upper block 16 which is in the direct sight of one viewing the display device. is transparent or clear. The block includes, in a first surface 20 (FIG. 2) thereof, a plurality of fluid receiving reservoirs or cavities, such as 22a, 22b, 220. In the particular display device shown in the drawing, three cavities are provided for each element 14 of the device. A decimal point element 24 (See FIG. 3) is also provided in the display device which includes only two cavities, however. This element will be explained in greater detail hereinafter.

In the preferred embodiment of the fluid display device 12, the cavities for each element 14, are interconnected for communication therebetween. The central cavity 22b is joined to the outer cavities 220, 220 by channels 26, 28 having predeterminedly sized openings formed at adjacent sides and extending along the length of the cavities to permit fluid to flow therebetween. The cavities are also interconnected by other channels or passages such as 23 also having predeterminedly sized openings extending from non-adjacent sides or ends 30, 32 and 34, 36 of the outer cavities 22a, 22c, respectively, to the ends 38, 40 of the central cavity 2211 (See FIG. 3). The volume of fluid containable in the central cavity 22b is substantially equal to that containable in both of the outer cavities 22a, 22c. Because the depths of the cavities is the same in the block 16, cavity 22b has been formed dimensionally larger in area than the other cavities (See FIG. 3). The passages 23, provide a return path for the fluids within the display element to promote a circuitous flow of fluid between the cavities. This provides a speedier exchange of fluid therebetween to in turn change the character represented by the display device rapidly.

The other block I8 is opaque and includes, along a first surface 42 thereof, a light reflecting strip 44. The surface 42 of the block 18 is joined to the surface 20 of block 16, with the strip 44 being aligned with the central cavity 22b thereof. The strip covers approximately the same area as the cavity 22b.

At the opposite surface 46 of block 18, there is provided, for each element, an electromagnet assembly 48. The assembly is mounted in a recess, such as 50, provided in surface 46 of the block 18 and includes a magnetic core of iron or the like material, having a main body 52 anda plurality of, herein four, core legs 54a, 54b, 54c, 54d extending therefrom. The number of core legs in the electromagnet assembly is one greater than the number of cavities included in the element. Thus, as will be seen hereinafter, for the two cavity decimal point element, three core legs are provided on the core body. Windings or coils 56a, 56b, 56c, are wound about the main body 52 of the core between the legs as shown, (See FlG. 2). The number of coils is equal to the number of cavities in the element.

The preferred fluid used in each element of the display device includes two liquids, the first of which'is an opaque liquid 58 having magnetic particles suspended therein. The opaque liquid has a predetermined surface tension characteristic, similar to that of water. The other liquid 60 is clear and immiscible; i.e. non-mixing. The clear liquid has the characteristic of wetting the surface of the block material forming the cavities to enhance the movement of the opaque magnetic liquid therebetween without breaking apart. The clear liquid is kerosene or a similar, suitable liquid. For a more detailed description of the magnetic fluid used in the display device according to the invention, see U.S. Pat. No. 3,648,269.

By selective energization of the coils 56a-56c from a suitable electrical energy source (not shown) which can be either a DC or an AC source, predetermined magnetic fields are produced to drive the magnetic liquid between cavities. When the magnetic liquid flows between the cavities, the clear liquid is forced out of the cavity location in which it had been maintained and into the cavity location from which the magnetic liquid came.

To illustrate the operation of the liquid display device according to the invention, one element 14 shown in the upper left hand corner of the display device of the various figures of the drawing, will be described.

Initially, as shown in FIGS. 4a, 4b, the central cavity 22b of the element 14 is with the clear immiscible liquid 60 and none of the windings 56a-56c is energized. In this condition, the light reflecting strip or area 44 on the surface 42 of block 18 can be viewed through the transparent block 16 (See FIG. 4b). The element thus assumes a first state or condition. The opaque magnetic liquid in the other cavities 22a, 22c, while also being seen through the transparent block 16, is not apparent to one viewing the block since the magnetic liquid and the opaque block are of the same color and thus blend together.

To move the magnetic liquid into the central cavity 22b to block the light reflecting strip 44 from the sight of one viewing the display device. the central winding 56b is energized. When energized, the core legs take on the polarities shown in FIGS. 5b, 50 to attract the magnetic liquid from the side cavities 22a, 22c for movement toward the central cavity 22b as shown. As the magnetic liquid 58 is moved between the channels 26, 28 provided between the cavities, into the central cavity, the clear liquid is forced outwardly therefrom as shown by the arrows in FIG. 5a, toward the ends 38, 40 of the central cavity into passages 23. The passages 23 provide quick removal of the clear liquid from the central cavity to permit the latter to fill quickly with the magnetic liquid. If the passages 23 were not provided. as in the case of prior art arrangements, all movement of the liquids would be along the channels 26, 28. The transfer thus would require an increased time because of the interaction between the liquids in the channels. In the case of the arrangement of the subject invention. no adjacent movement of the liquids in opposite directions takes place.

Once the liquids have assumed their new positions, in the second state or condition, they remain therein without the need for continuing the energization of the central winding 56b. With the opaque magnetic fluid covering the reflective portion 44 on the block 18, one observing the display device will not see the light reflecting strip 44. Likewise, one viewing the device will see in cavities 2211, 220 only the opaque block 18 therethrough. The element 14 in effect, disappears from view.

The display device is bistable; i.e. it will remain in either of the two states described without having power applied to the coils. The latter is accomplished by providing the predeterminedly sized openings in channels 26, 28 and passages 23, which prevent the magnetic liquid 58 from flowing thereinto without the application of a force applied thereto. The surface tension of the magnetic liquid causes the liquid to tend to take on a spherical shape, such as a droplet of water. With the channels and passages being relatively small in size, the surface tension of the magnetic liquid can in normal use support the liquid thereagainst without flowing through. An illustration of the channel width is provided in FIG. 2 and is shown by the dimension x between an imaginary line y representing the face or surface 20 of the block 16 and the edge 1 of the block material between cavities 22a, 22b. During the operation of the display device it is intended that no magnetic liquid flow through passages 23, the latter being intended to carry only the clear immiscible liquid. The actual dimensions of the channels and the passages are dictated by the surface tension characteristics of the magnetic fluid, and, as such, can vary depending upon the latter. These dimensions can readily be provided, however, by one skilled in the art.

To return the element 14 to its initial state, coils 56a, 560 are energized. Upon energization of the lastmentioned coils, the core legs take on the polarities shown in FIGS. 7b, 7c to both repel the magnetic liquid from the central cavity 22b and attract the magnetic liquid to the outer cavities 22a, 220 as shown in FIG. 7a. The surface tension of the magnetic liquid is overcome and the magnetic liquid is forced through channels 26, 28 from the central cavity 22b into the outer cavities 22a, 22c and the clear or transparent liquid is returned to the central cavity by the force of the magnetic liquid thereagainst, via passages 23. Once the magnetic liquid has returned fully to the outer cavities 22a, 22c, the coils may be deenergized and the liquids remain in the last-mentioned respective cavities regardless of the positioning of the device.

As mentioned heretofore, a decimal point element 24 is also provided in each of the display devices 12 according to the invention. Referring to FIGS. 4a, 8 and 9 of the drawings, it can be seen that the decimal point element comprises a pair of equally dimensioned cavities 70a, 70b which are also formed in surface of the plastic block 16, similarly to the elements 14 formed therein. The cavities 70a, 70b are of similar volumes and are joined by a channel 72 formed along adjacent edges thereof and by passages or channels 74 which extend between nonadjacent sides of the cavities to provide a circuitous fluid path.

As in the case of the elements 14, the decimal point element is provided with an electromagnet assembly 78 (See FIGS. 8 and 9) which is mounted in a recess 80 formed in surface 46 of the opaque block 18. In the case of the decimal point element, the clectromagnet assembly requires only two coils or windings 82a, 82b, which are wound about the main body 84 of the magnetic core 86 of the electromagnet assembly, between respective core legs 88a, 88b, 88c extending from the main body.

Also, as in the case of the element 14, a light reflecting spot 90 is provided on the surface 42 of the opaque block 18 which is aligned with one, herein 70b, of the cavities comprising the element. A quantity of magnetic liquid 58 is included in the decimal point element which is sufficient to fill one of the cavities. Clear liquid 60 fills the remaining volume including the other cavity and the passages 74.

Upon energization of coil 82a, the adjacent core legs are poled as shown in FIG. 8 to attract the magnetic liquid 58 in cavity 70a, causing the liquid to flow through the channel 72 into cavity 70b. Simultaneously therewith, the clear liquid 60 is forced out of cavity 7012 through passages 74 into cavity 70a. When all of the magnetic liquid 58 has been transferred to cavity 70b, the liquid tends to form a sphere with the surface tension thereof preventing the liquid from flowing out of the cavity even after coil 82a has been deenergized.

With the magnetic liquid 58 in cavity 70b as shown in FIG. 6a, the light reflecting spot (See FIG. 8) is covered and so the element is not visible by one viewing the liquid display device.

To return the magnetic liquid to cavity 70a, the other winding or coil 82b is energized (See FIG. 9). The energization thereof sets up magnetic poles in the core legs as shown which produces a magnetic field to attract the magnetic liquid 58, causing it to be moved through channel 72 from cavity 70b to cavity 70a, thereby revealing the light reflecting spot 90. The clear liquid again is forced to flow through the passages 74 from cavity 700 into cavity 70b (See FIG. 7a).

In addition to providing a decimal point element, a second element (not shown) may be provided at an upper location on the display device which can be used in conjunction with the decimal point element to provide a colon or the like symbol for use in indicating time".

The display device according to the invention is simple in design, yet efficient in operation, and because no energy is required to maintain the device in one or the other of its two stable states or conditions, is relatively inexpensive to operate.

The display device of the invention has other uses than merely as a visual display device. Because the relative permeability of the core portions of the core of the electromagnet assembly of any one of the elements varies in accordance with the placement of the magnetic liquid droplet, the state or condition of the device can be determined even without viewing. To determine the state of the device; i.e. to ascertain whether the reflecting strip or spot is visible or hidden, one need only supply a small alternating current from a generator, such as shown in FIGS. 4!), 6h, through the coil wrapped about one of the core sections, preferably the core section adjacent the cavity 22b covering the strip and a current sensing device 102 of the type well known in the art. If the magnetic liquid is present in the cavity, the permeability of the core section will be different than if no magnetic liquid is present. This will change the inductance of the coil and thus its impedance. This difference in impedance can be sensed by suitable circuitry including the current source 100 and current sensing device 102 which can be provided in an electronic calculator or the like machine, and thus a reading can be made without actually viewing the display arrangement.

In addition, because each device remains in the state in which it has been set, an inherent memory" is provided also. The device therefore lends itself for use in calculator and the like machines which employ numerical displays and which often require the storing of information which is to be used at a later time.

While a particular embodiment of the invention has been shown and described, it should be understood that the invention is not limited thereto since many modifications may be made. One such modification may be the provision of an aperture through block 18 in place of the light reflecting strip. Through the use of a fiber optic bundle, light tube or the like, a light source may be employed to indicate the state or condition of the element. It is therefore contemplated to cover by the present application any and all such modifications as fall within the true spirit and scope of the appended claims.

I claim:

1. A fluid display device including a housing having means defining a pair of cavities therein, means defining a channel connecting said cavities for communication therebetween, an opaque fluid having a predetermined surface tension characteristic provided in one of said cavities, a clear fluid provided in the other one of said cavities, said opaque fluid including magnetic particles suspended therein and said clear fluid being immiscible, and magnetic field producing means mounted adjacent said housing for providing a magnetic field of a predetermined strength to cause said magnetic fluid and clear fluid to be moved through said channel into the other of said cavities, respectively, said channel having a predeterminedly sized opening such that said opaque magnetic fluid having said predetermined surface tension characteristic remains in the cavity into which it has been moved despite the removal of the predetermined magnetic field by said magnetic field producing means and the position of said housing.

2. A fluid display device as claimed in claim 1 wherein said channel means is provided along adjacent, opposing sides of said cavities and wherein said housing further include means defining a separate passage extending between non-adjacent sides of said cavities so that upon application of said magnetic field, the magnetic fluid is transferred between cavities through one of said channel and passage and said clear fluid is transferred between cavities through the other one of said channel and passage.

3. A fluid display device as claimed in claim 2 wherein said magnetic field producing means includes an electromagnet assembly having a core of magnetizable material with at least three core legs extending therefrom and first and second coils each wound about said core between a pair of said core legs, the energization of said first coil causing said first and second core legs to be polarized oppositely, thereby to produce a first magnetic field for moving said magnetic fluid from a first one of said cavities to the other of said cavities and the energization of said second coil causing said second and third legs to be polarized oppositely, thereby to produce a second magnetic field for moving said magnetic fluid from said other cavity to said first one of said cavities.

4. A fluid display device as claimed in claim 2 wherein said housing further includes means defining a third cavity provided adjacent one of said pair of cavities, means defining a second channel dimensioned similarly to said first-mentioned channel formed between said third cavity and said one cavity of said pair for communication therebetween and means defining a second passage, separate from said channels and said first-mentioned passage, extending between said one cavity of said pair of cavities and said third cavity, wherein said one cavity of said pair is substantially equal in volume to the sum of the volumes of said other cavity of said pair and said third cavity, wherein the volume of magnetic fluid provided in said device is equal to that required to fill said one cavity of said pair and wherein said magnetic field producing means provides a first magnetic field to transfer the magnetic fluid out of said one cavity of said pair through said channels into the other of said cavities of said pair and said third cavity, respectively, and second and third magnetic fields to transfer said magnetic fluid back from said other of said pair of cavities and said third cavity to said one cavity of said pair, said clear fluid being transferred simultaneously between said one cavity of said pair and said other one of said cavities of said pair and said third cavity, through said passages.

5. A fluid display device as claimed in claim 1 wherein said housing comprises a block of transparent material and wherein said means defining said pair of cavities in said housing further include a light reflecting wall portion mounted adjacent a first one of said cavities and a non-reflecting wall portion adjacent the second one of said pair of cavities, so that upon moving said opaque fluid from said first to said second one of said pair of cavities, said light reflecting wall portion is visible to one viewing said fluid display device.

6. A fluid display device including in combination: a housing formed of a first transparent block portion and a second opaque block portion joined together along first surfaces thereof, at least one display element provided in said housing, said display element including means defining a plurality of cavities formed in the first surface of said transparent block portion, means defining channels between adjacent sides of predetermined ones of said cavities, respectively, formed in the first surface of said transparent block portion, means defining passages between non-adjacent sides of said predetermined ones of said cavities formed in the first surface of said transparent block portion, means defining a visible light emitting area formed along the first surface of said opaque block portion in alignment with a predetermined one of said cavities formed in said transparent block portion and being visible through said last-mentioned block portion, an opaque fluid including magnetic particles suspended therein having a predetermined surface tension characteristic provided in predetermined ones of said cavities, a clear fluid provided in the other of said cavities, said display element thereby assuming a first stable condition, and magnetic field producing means mounted adjacent said housing for providing a magnetic field of a predetermined strength for transferring said magnetic fluid through said channels from said predetermined ones of said cavities to other predetermined ones of said cavities and simultaneously to transfer said clear fluid through said passages from said other cavities to predetermined other ones of said cavities, thereby to operate said display element to a second stable condition, said opaque fluid being received in said one predetermined cavity for blocking the view of said light emitting area in one of said stable conditions and being removed therefrom to reveal said light emitting area in the other of said stable conditions.

7. A fluid display device as claimed in claim 6 wherein said one display element includes first, second and third cavities, said first and second cavities being joined between adjacent edges thereof by a first channel and the second and third cavities being joined between adjacent edges thereof by a second channel, said first and second cavities also being joined between nonadjacent edges thereof by a first separate passage and said second and third cavities also being joined between non-adjacent edges thereof by a second separate passage thereby providing a circuitous path for the flow of said fluids, wherein the sum of the volume of said first and third cavities is substantially equal to the volume of said second cavity, wherein said opaque fluid is provided in said first and third cavities in said first stable state and in said second cavity in said second stable state, wherein said light emitting area is formed along the first surface of said opaque block portion in alignment with said second cavity and wherein said magnetic field producing means includes an electromagnet assembly, said electromagnet assembly being energizable to produce a first magnetic field means for transferring said opaque fluid through said first and second channels from said first and third cavities to said second cavity, said clear fluid being transferred from said second cavity through said passages to said first and third cavities, respectively, and to produce a second magnetic field means for transferring said opaque fluid through said first and second channels from said second cavity to said first and third cavities, said clear fluid being transferred through said first and second passages from said first and third cavities to said second cavity.

8. A fluid display device as claimed in claim 7 wherein the second cavity of said display element is elongated in shape and wherein said device includes seven of said display elements positioned to form a figure eight symbol, said device displaying numerals from O to 9 depending upon the condition of said seven display elements.

9. A fluid display device as claimed in claim 6 wherein said display element includes first and second cavities joined between adjacent edges thereof by a channel formed therebetween, and joined at nonadjacent edges thereof by a separate passage to provide a circuitous path for the flow of said fluids, wherein said cavities are substantially equal in volume, wherein said opaque fluid is provided in said first cavity in said first stable state and in said second cavity in said second stable state, wherein said light emitting area is formed along the first surface of said opaque block portion in alignment with said second cavity and wherein said magnetic field producing means includes an electromagnet assembly energizable to produce a first magnetic field for transferring said opaque fluid through said channel from said first to said second cavity, said clear fluid being transferred from said second to said first cavity through said passage, and to produce a second magnetic field to transfer said opaque fluid through said channel from said second to said first cavity, said clear fluid being transferred through said passage from said first to said second cavity.

10. A fluid display device as claimed in claim 6 wherein said magnetic field producing means includes an electromagnet assembly comprising magnetizable core means having a plurality of core legs extending therefrom toward said fluid receiving cavities, the number of core legs being one greater than the number of fluid filled cavities, so that a pair of said core legs is positioned adjacent each one of said cavities and a plurality of coils wound about said core between said core legs. predetermined ones of said coils being energizable to produce first magnetic field means to operate said display element to said first stable condition and other predetermined ones of said coils being energizable to produce second magnetic field means to operate said display element to said second stable state.

11. A fluid display device as claimed in claim 10 further including means for sensing the condition of said display element, said means including impedance sensing means having an alternating current source coupled to a predetermined one of said coils and current sensing means, said impedance sensing means indicating the change in condition of said coil depending upon the location of said opaque magnetic fluid with respect to said associated core legs.

12. A fluid display device as claimed in claim 7 wherein said electromagnet assembly includes a magnetizable core body mounted on said opaque block adjacent said cavities, said core including first, second, third and fourth core legs extending therefrom, said first and second core legs being located adjacent said first cavity, said second and third core legs being located adjacent said second cavity and said third and fourth core legs being located adjacent said third cavity, and three coils wound about said core each between a pair of said core legs, said first coil wound between said first and second core legs, said second coil wound between said second and third core legs and said third coil wound between said third and fourth core legs, said second coil being energized to produce opposite polarity poles at said second and third core legs to create a magnetic field for attracting said magnetic fluid from said first and third cavities toward said second cavity and said first and third coils being energized to produce opposite polarity poles at said first and second, and third and fourth core legs, respectively, with said second and third core legs being of like polarity, to create magnetic fields for moving said magnetic liquid from said second cavity to said first and third cavities, respectively.

13. A fluid display device as claimed in claim 9 wherein said electromagnet assembly includes a magnetizable core body mounted on said opaque block adjacent said cavities, said core including first, second and third core legs extending therefrom, said first and second core legs being positioned adjacent said first cavity and said second and third core legs being positioned adjacent said second cavity, and first and second coils wound about said core, each between a pair of said core legs, said first coil being wound between said first and second core legs and said second coil being wound between said second and third core legs, said first coil being energized to produce opposite polarity poles at said first and second core legs to create a first magnetic field for attracting said magnetic fluid to move the latter from said second to said first cavity and said second coil being energized to produce opposite polarity poles at said second and third core legs to create a second magnetic field for attracting said magnetic fluid, to move the latter from said first to said second cavity. 

1. A fluid display device including a housing having means defining a pair of cavities therein, means defining a channel connecting said cavities for communication therebetween, an opaque fluid having a predetermined surface tension characteristic provided in one of said cavities, a clear fluid provided in the other one of said cavities, said opaque fluid including magnetic particles suspended therein and said clear fluid being immiscible, and magnetic field producing means mounted adjacent said housing for providing a magnetic field of a predetermined strength to cause said magnetic fluid and clear fluid to be moved through said channel into the other of said cavities, respectively, said channel having a predeterminedly sized opening such that said opaque magnetic fluid having said predetermined surface tension characteristic remains in the cavity into which it has been moved despite the removal of the predetermined magnetic field by said magnetic field producing means and the position of said housing.
 2. A fluid display device as claimed in claim 1 wherein said channel means is provided along adjacent, opposing sides of said cavities and wherein said housing further include means defining a separate passage extending between non-adjacent sides of said cavities so that upon application of said magnetic field, the magnetic fluid is transferred between cavities through one of said channel and passage and said clear fluid is transferred between cavities through the other one of said channel and passage.
 3. A fluid display device as claimed in claim 2 wherein said magnetic field producing means includes an electromagnet assembly having a core of magnetizable material with at least three core legs extending therefrom and first and second coils each wound about said core between a pair of said core legs, the energization of said first coil causing said first and second core legs to be polarized oppositely, thereby to produce a first magnetic field for moving said magnetic fluid from a first one of said cavities to the other of said cavities and the energization of said second coil causing said second and third legs to be polarized oppositely, thereby to produce a second magnetic field for moving said magnetic fluid from said other cavity to said first one of said cavities.
 4. A fluid display device as claimed in claim 2 wherein said housing further includes means defining a third cavity provided adjacent one of said pair of cavities, means defining a second channel dimensioned similarly to said first-mentioned channel formed between said third cavity and said one cavity of said pair for communication therebetween and means defining a second passage, separate from said channels and said first-mentioned passage, extending between said one cavity of said pair of cavities aNd said third cavity, wherein said one cavity of said pair is substantially equal in volume to the sum of the volumes of said other cavity of said pair and said third cavity, wherein the volume of magnetic fluid provided in said device is equal to that required to fill said one cavity of said pair and wherein said magnetic field producing means provides a first magnetic field to transfer the magnetic fluid out of said one cavity of said pair through said channels into the other of said cavities of said pair and said third cavity, respectively, and second and third magnetic fields to transfer said magnetic fluid back from said other of said pair of cavities and said third cavity to said one cavity of said pair, said clear fluid being transferred simultaneously between said one cavity of said pair and said other one of said cavities of said pair and said third cavity, through said passages.
 5. A fluid display device as claimed in claim 1 wherein said housing comprises a block of transparent material and wherein said means defining said pair of cavities in said housing further include a light reflecting wall portion mounted adjacent a first one of said cavities and a non-reflecting wall portion adjacent the second one of said pair of cavities, so that upon moving said opaque fluid from said first to said second one of said pair of cavities, said light reflecting wall portion is visible to one viewing said fluid display device.
 6. A fluid display device including in combination: a housing formed of a first transparent block portion and a second opaque block portion joined together along first surfaces thereof, at least one display element provided in said housing, said display element including means defining a plurality of cavities formed in the first surface of said transparent block portion, means defining channels between adjacent sides of predetermined ones of said cavities, respectively, formed in the first surface of said transparent block portion, means defining passages between non-adjacent sides of said predetermined ones of said cavities formed in the first surface of said transparent block portion, means defining a visible light emitting area formed along the first surface of said opaque block portion in alignment with a predetermined one of said cavities formed in said transparent block portion and being visible through said last-mentioned block portion, an opaque fluid including magnetic particles suspended therein having a predetermined surface tension characteristic provided in predetermined ones of said cavities, a clear fluid provided in the other of said cavities, said display element thereby assuming a first stable condition, and magnetic field producing means mounted adjacent said housing for providing a magnetic field of a predetermined strength for transferring said magnetic fluid through said channels from said predetermined ones of said cavities to other predetermined ones of said cavities and simultaneously to transfer said clear fluid through said passages from said other cavities to predetermined other ones of said cavities, thereby to operate said display element to a second stable condition, said opaque fluid being received in said one predetermined cavity for blocking the view of said light emitting area in one of said stable conditions and being removed therefrom to reveal said light emitting area in the other of said stable conditions.
 7. A fluid display device as claimed in claim 6 wherein said one display element includes first, second and third cavities, said first and second cavities being joined between adjacent edges thereof by a first channel and the second and third cavities being joined between adjacent edges thereof by a second channel, said first and second cavities also being joined between non-adjacent edges thereof by a first separate passage and said second and third cavities also being joined between non-adjacent edges thereof by a second separate passage thereby providing a circuitous path for the flow of said fluids, wherein tHe sum of the volume of said first and third cavities is substantially equal to the volume of said second cavity, wherein said opaque fluid is provided in said first and third cavities in said first stable state and in said second cavity in said second stable state, wherein said light emitting area is formed along the first surface of said opaque block portion in alignment with said second cavity and wherein said magnetic field producing means includes an electromagnet assembly, said electromagnet assembly being energizable to produce a first magnetic field means for transferring said opaque fluid through said first and second channels from said first and third cavities to said second cavity, said clear fluid being transferred from said second cavity through said passages to said first and third cavities, respectively, and to produce a second magnetic field means for transferring said opaque fluid through said first and second channels from said second cavity to said first and third cavities, said clear fluid being transferred through said first and second passages from said first and third cavities to said second cavity.
 8. A fluid display device as claimed in claim 7 wherein the second cavity of said display element is elongated in shape and wherein said device includes seven of said display elements positioned to form a figure eight symbol, said device displaying numerals from 0 to 9 depending upon the condition of said seven display elements.
 9. A fluid display device as claimed in claim 6 wherein said display element includes first and second cavities joined between adjacent edges thereof by a channel formed therebetween, and joined at non-adjacent edges thereof by a separate passage to provide a circuitous path for the flow of said fluids, wherein said cavities are substantially equal in volume, wherein said opaque fluid is provided in said first cavity in said first stable state and in said second cavity in said second stable state, wherein said light emitting area is formed along the first surface of said opaque block portion in alignment with said second cavity and wherein said magnetic field producing means includes an electromagnet assembly energizable to produce a first magnetic field for transferring said opaque fluid through said channel from said first to said second cavity, said clear fluid being transferred from said second to said first cavity through said passage, and to produce a second magnetic field to transfer said opaque fluid through said channel from said second to said first cavity, said clear fluid being transferred through said passage from said first to said second cavity.
 10. A fluid display device as claimed in claim 6 wherein said magnetic field producing means includes an electromagnet assembly comprising magnetizable core means having a plurality of core legs extending therefrom toward said fluid receiving cavities, the number of core legs being one greater than the number of fluid filled cavities, so that a pair of said core legs is positioned adjacent each one of said cavities and a plurality of coils wound about said core between said core legs, predetermined ones of said coils being energizable to produce first magnetic field means to operate said display element to said first stable condition and other predetermined ones of said coils being energizable to produce second magnetic field means to operate said display element to said second stable state.
 11. A fluid display device as claimed in claim 10 further including means for sensing the condition of said display element, said means including impedance sensing means having an alternating current source coupled to a predetermined one of said coils and current sensing means, said impedance sensing means indicating the change in condition of said coil depending upon the location of said opaque magnetic fluid with respect to said associated core legs.
 12. A fluid display device as claimed in claim 7 wherein said electromagnet assembly includes a magnetizable corE body mounted on said opaque block adjacent said cavities, said core including first, second, third and fourth core legs extending therefrom, said first and second core legs being located adjacent said first cavity, said second and third core legs being located adjacent said second cavity and said third and fourth core legs being located adjacent said third cavity, and three coils wound about said core each between a pair of said core legs, said first coil wound between said first and second core legs, said second coil wound between said second and third core legs and said third coil wound between said third and fourth core legs, said second coil being energized to produce opposite polarity poles at said second and third core legs to create a magnetic field for attracting said magnetic fluid from said first and third cavities toward said second cavity and said first and third coils being energized to produce opposite polarity poles at said first and second, and third and fourth core legs, respectively, with said second and third core legs being of like polarity, to create magnetic fields for moving said magnetic liquid from said second cavity to said first and third cavities, respectively.
 13. A fluid display device as claimed in claim 9 wherein said electromagnet assembly includes a magnetizable core body mounted on said opaque block adjacent said cavities, said core including first, second and third core legs extending therefrom, said first and second core legs being positioned adjacent said first cavity and said second and third core legs being positioned adjacent said second cavity, and first and second coils wound about said core, each between a pair of said core legs, said first coil being wound between said first and second core legs and said second coil being wound between said second and third core legs, said first coil being energized to produce opposite polarity poles at said first and second core legs to create a first magnetic field for attracting said magnetic fluid to move the latter from said second to said first cavity and said second coil being energized to produce opposite polarity poles at said second and third core legs to create a second magnetic field for attracting said magnetic fluid, to move the latter from said first to said second cavity. 